1. Field of the Invention
The present invention relates to an optical fiber assembly such as an optical fiber connector plug and, more particularly, to an optical fiber assembly which is expected to be used under a severe environmental condition wherein temperature frequently changes over a wide range with high and low extreme temperatures.
2. Description of the Related Art
An optical fiber assembly is often used under a severe environmental condition wherein temperature frequently changes greatly between high and low extremes. The following describes problems found in such an optical fiber assembly.
FIG. 7 is a cross-sectional view showing a conventional optical fiber connector plug; FIG. 8 is a cross-sectional view illustrating an optical fiber connector plug which has been broken during a heating cycle test. A zirconia ceramic ferrule is extensively used for a ferrule 3 which has a through hole for receiving an optical fiber 2 at the center of the ferrule 3. A metallic supporting sleeve 4 made of stainless steel has a flange, a stepped hole 6 for receiving the base of the ferrule 3, and a hole for receiving an optical fiber protective sheath 1.
The optical fiber protective sheath 1 employs UV resin or PVC resin. The optical fiber 2 is fixed in a hole 5 of the ferrule 3 with an epoxy resin adhesive agent 17a; the optical fiber protective sheath 1 is glued and fixed on the metallic supporting sleeve 4 with epoxy resin adhesive agents 17b, 17c.
Various tests are conducted, expecting that the optical fiber assembly will be used in a severe environment. The upper limit of the temperature range which an optical connector is required to survive during heating cycle test is +80.degree. C. and the lower limit is -40.degree. C. During the heating cycle test, the temperature is increased from 25.degree. C. to the upper limit, namely, +80.degree. C. in 30 minutes, then the upper limit temperature is maintained for 30 minutes; the temperature is then decreased back to 25.degree. C. in 30 minutes, and the 25.degree. C. temperature is maintained for 30 minutes before it is decreased to the lower limit, namely, -40.degree. C., in 30 minutes; then the -40.degree. C. temperature is maintained for 30 minutes before it is increased back to 25.degree. C. in 30 minutes and the 25.degree. C. temperature is maintained for 30 minutes. This cycle is repeated. The number of repetitions ranges from 10 to 1000 cycles although it depends on the request of each user. In the heating cycle test, the optical characteristics of the optical connector are required to stay within specified values.
The heating cycle test is one of the extremely exacting tests for a device with an optical fiber; there has been a problem in that an optical fiber breaks during a heating cycle test which consists of many repeated cycles of 100 or more.
Specifically, as shown in FIG. 8, it often happens that the optical fiber 2 breaks at point B and the optical fiber sheath 1 moves out in the direction of the arrow from the stepped hole 6 of the supporting sleeve 4.
The problem is considered attributable mainly to the difference in expansion from the difference in the linear expansion coefficient based on the different materials used for the metallic supporting sleeve 4 and the optical fiber protective sheath 1, the transition of the epoxy resin adhesive agent into hard glass due to curing, the change which takes place in the hardness of the protective sheath made of hard-to-stick PVC resin, and unsuccessful adhesion to the protective sheath 1 which has a smooth surface. When stainless steel is used for the metallic supporting sleeve 4, the linear expansion coefficient, which constitutes a factor for the shift in the axial direction, will be approximately 12.times.10.sup.-6 /.degree.C. In the case of a single-mode (SM) optical fiber core with a quartz (Si) type clad, the linear expansion coefficient is 12.times.10.sup.-6 /.degree.C. which is practically similar to the one obtained with the stainless steel. Using the PVC resin, however, for the optical fiber protective sheath 1 causes a linear expansion coefficient, which constitutes a factor for the shift in the axial direction, to be 50.times.10.sup.-6 /.degree.C., indicating a significant difference between the core and the stainless steel.
If an adhesion length L of the protective sheath 1 is 5 mm and the difference in temperature during the heating cycle test is 120.degree. C., then the expansion of the protective sheath 1 will be 30 .mu.m in relation to the axial expansion 7 .mu.m of the metallic supporting sleeve 4, the difference in expansion being approximately 23 .mu.m. Hence the protective sheath 1, which is incompletely glued at high temperature, extends out of the supporting sleeve 4 by the length which corresponds to the difference in expansion between the protective sheath 1 and the flange. It is considered that the protective sheath 1 moves to the right in FIG. 7 and FIG. 8 because it is difficult for it to move toward the ferrule.
On the contrary, the protective sheath 1 shrinks at low temperature; however, it does not go back to home position thereof. This is considered because the PVC resin hardens at the extremely low temperature, namely, -40.degree. C. and the frictional force or the like produced between the protective sheath 1 and the surface of the ferrule hole prevents the protective sheath 1 from going back to the home position thereof.
Thus, the PVC resin protective sheath 1 comes out in a very small amount by the difference in expansion for each cycle.
Hence, it is presumed that tensile stress in the axial direction is accumulated in the optical fiber 2 for each cycle until the optical fiber 2 finally breaks. FIG. 8 shows the optical fiber 2 which has broken with the protective sheath 1 drawn together with the adhesive agent 17c out of the hole of the supporting sleeve 4. The phenomenon where a part of the optical fiber slips out is called "pistoning phenomenon."
The breakage of the optical fiber stated above is a critical shortcoming of an optical fiber assembly. Repeated application of excessive stress to a part of the optical fiber can cause deterioration in the optical characteristics of the optical fiber even if the optical fiber does not break.